I don’t normally have a lot to do with quantum foundations, especially not since I left Perimeter Institute. And so I learned many new things and got feedback on my paper. It was a useful meeting for me – but it was also a little strange.

Most of the feedback I got was people telling me they don’t believe in superdeterminism, wanting to know why I believe in it, not that I’m sure I do. Discussions turned towards final causes and theology. I’m a phenomenologist, I heard myself saying, I couldn’t care less what other people believe, I want to know how it can be tested. Faintly, I heard an echo of a conversation I had with Joao Magueijo at PI some years ago. Boy, I thought back then, does this guy get explosive when asked about his beliefs. Now I think he must have been spending too much time with the quantum foundations folks. Suddenly I’m very sympathetic to Joao’s attitude.

Quantum foundations polarizes like no other area in physics. On the one hand there are those actively participating who think it’s the most important thing ever but no two of them can agree on anything. And then there’s the rest who thinks it’s just a giant waste of time. In contrast, most people tend to agree that quantum gravity is worthwhile, though they may differ in their assessment of how relevant it is. And while there are subgroups in quantum gravity, there’s a lot of coherence in these groups (even among them, though they don’t like to hear that).

As somebody who primarily works in quantum gravity, I admit that I’m jealous of the quantum foundations people. Because they got data. It is plainly amazing for me to see just how much technological progress during the last decade has contributed to our improved understanding of quantum systems. May that be tests of Bell’s theorem with entangled pairs separated by hundreds of kilometers, massive quantum oscillators, molecule interferometry, tests of the superposition principle, weak measurements, using single atoms as a double slit, quantum error correction, or the tracking of decoherence, to only mention what popped into my head first. When I was a student, none of that was possible. This enables us to test quantum theory now much more precisely and in more circumstances than ever before.

This technological progress may not have ignited the interest in the foundations of quantum mechanics but it has certainly contributed to the field drawing more attention and thus drawing more people. That however doesn’t seem to have decreased the polarization of opinions, but rather increased it. The more attention research on quantum foundations gets, the more criticism it draws.

“Shut up and let me think” is the title of an essay by Pablo Echenique-Robba which you can find on the arxiv at 1308.5619 [quant-ph]. In his personal account Pablo addresses common arguments for why research on quantum foundations is a waste of time. I’ve encountered most of these and I largely agree with his objections. But let me add some points Pablo didn’t mention.

I do have my issues with much of what I’ve seen in quantum foundations. To begin with, most of it seems to be focused on non-relativistic quantum mechanics. That’s like trying to improve the traffic in NYC by breeding better horses. If you can’t make it Lorentz-invariant and second quantized I don’t know why I should think about it. More important, I can’t fathom what most of the interpretation-pokers are aiming at. It’s all well and fine with me to try to find another formulation for the theoretical basis of quantum theory. But in the end I want to see either exactly what the observable differences are or I want to see a proof of equivalence. Alas, there seems to be a lot of talk about, well, interpretations which do neither one nor the other. Again the phenomenologist lacks the motivation to think about it.

Despite these reservations I think that research on the foundations of quantum mechanics is of value, again for a reason that Pablo did not address in his paper, so I want to add.

I’ve been educated in the “shut up and calculate” philosophy with my profs preaching Feynman’s mantra that nobody understands quantum mechanics, so don’t bother trying. Needless to say I, as probably most students, was not so much deterred as encouraged by this, so we dug a little into the literature. If you dig, it gets into philosophy very quickly. That’s not necessarily a bad thing, but most students come around to realize they wanted to study physics, not philosophy, and they move on to calculate. I’m among those who feel comfortable with a mathematical framework that “just” delivers results and that can be used to describe nature. To me science is “just” about making good models.

But those who are criticizing research on the foundations of quantum mechanics on the ground that everything has been understood are dismissing a way to arrive at an improved description of nature, and they are dismissing it based on unjustified arrogance about their superior motives.

Science progresses by evaluating the use of models about nature in the form of specific hypotheses. What we call ‘scientific method’ are procedures that have proved efficient in creating good hypotheses and tests thereof. Not only do these methods change (hopefully improve) over time, what constitutes a ‘good’ hypothesis also depends on beliefs and social dynamics. In the end what matters is not how somebody arrived at a hypothesis, but whether it works. That’s the essence of scientific progress.

The action principle, gauge-symmetry, and unification, for example, have proved dramatically useful in the construction of theories. And that they have been useful in the past is a good reason to employ them in the future search for improved theories. The same goes for naturalness. A theory that isn’t ‘natural’ is typically believed to be incomplete and in need of improvement or at least additional explanation. Yet all that says is that it’s a criterion which researchers draw upon to arrive at better theories. There’s no proof that this will work. It’s a reasonable guess, that’s all. How reasonable depends on your attitude, your beliefs and on whether you think it’ll land you a job.

And so some may guess there is something to be gained by poking around on the foundations of quantum mechanics. You might not believe that the reasons for their interest are good reasons, much like I don’t believe in naturalness and others don’t believe in a theory of everything. But in the end it doesn’t matter. In the end what matters is not what motivated people to study some research question, but only whether it led to something.

My support for quantum foundations thus comes from a live-and-let-live attitude. Maybe studying the foundations of quantum theory will improve our understanding of the fundamental nature of reality. Maybe it won’t. I don’t understand most of their motivations. But then they don’t understand mine either.

Those who are dismissing quantum foundations as a waste of time I want to ask to consider the consequences of this research in fact revealing a different theory underlying quantum mechanics, one that allows us to manipulate quantum processes in novel ways. The potential is enormous. It’s not a stone that should be left unturned.

61 comments:

I haven't had a chance to read your paper on superdeterminism yet, but I look forward to it. It sounds similar to some ideas 't Hooft has toyed with in the past (avoiding Bell's prohibition on local deterministic theories by denying "free will"). I assume you're aware of his papers on this; if not, you'll probably want to look at them.

This isn't entirely in line with the symposium you attended but it's a rather elegant and interesting paper on the foundations of Quantum Theory: Quantum Theory and Probability Theory: Their Relationship and Origin in Symmetry (http://www.mdpi.com/2073-8994/3/2/171) by Philip Goyal and Kevin Knuth. I look forward to reading your superdeterminism paper.

First of all, thanks Sabine for the post and for introducing me to superdeterministic theories, which I didn't know existed (although maybe I was superdeterministically destined to know them through you ;) ). I don't believe in (strong) free will either, so the idea resonates with my impression of how the world is....Second, I did not get the conference bag and the umbrella! Are you sure they were for attendants too and not only for speakers?...Now, coming to your post and the main point in it: Although I am mostly a theoretical person, and very inclined into philosophical matters, I am also a pragmatist, and I know that experiments are the final word. Not only because they are the only thing that can reliably rule out false hypotheses but also because they help reduce the width of the theoretical speculation to narrower, more manageable sizes.

However, I also think that it is not possible to design the best experiments if one does not understand properly what the hell is it what one is trying to prove or disprove. As I say in my manuscript, and many scientists agree, our understanding of QM is so embarrassingly poor that it is probably preventing us from designing the best experiments. This is an historical accident due to the strong influence of the antirationalistic arguments by Bohr and followers which we now can correct thanks to the amazing new technical capabilities we have and also because working in foundations has stopped being taboo and a ticket out of academia.

I also agree (of course) that any proper version of QM has to be relativistic, but hey, give us some time! Orthodox QM wasn't relativistic at the beginning either. It took them decades to make it so. I know in fact that there are many promising roads open to make the most serious alternatives to orthodox QM (such as Bohmian mechanics and GRW) relativistic.

Finally, I think that one of the points in which a better understanding of QM could make a difference is when we interface QM with other theories. If one needs to use QM only to "apply" it, maybe knowing it at the level of "a recipe" would do. However, if one needs to know, say, how QM "talks" to general relativity, it is crucial to have a deep understanding of both theories in my opinion.

Thanks for your comment. Regarding the bag, uhm, I don't know. Somebody at the registration handed me one and I just assumed everybody got one.

I think 'embarrassingly poor' is an exaggeration. I actually think it's exaggerations like that which serve to undermine the credibility of the field you work in...

You probably misunderstood my comment on non-relativistic qm and testability. I understand that it can take time to get there. What I am missing is even the acknowledgement that this is what one wants to aim at or some, if so vague, idea of how to achieve it. This leaves me with the impression that people aren't even trying, but are contempt to re-investigate the double-slit experiment over and over again while the rest of the world does NNNLO calculations in QCD if you see what I mean.

I agree that "embarrassingly poor" is a bit of an exaggeration, but I am not the only one making it ;) http://www.preposterousuniverse.com/blog/2013/01/17/the-most-embarrassing-graph-in-modern-physics/ I think that I mean more or less the same as Carroll with this. I.e., it is not "embarrassingly poor" in the absolute sense, but it is indeed for a theory that has been around for almost a century now. And the reason is clear: from the 30s to the 60s (approx.) people thought that there was nothing to be understood. So they didn't try. From the 60s to, say, the 90s, the hangover started to slowly dissipate, but it was still dangerous for you to work in foundations if you wanted to earn a living. Now the tide is changing, but we lost many decades believing Bohr was right.

As for people not worrying about relativity, I disagree. In this interview, for example, you have a very philosophically oriented guy stating that the largest theoretical problem is to combine Bohmian mechanics with relativity http://www.youtube.com/watch?v=nQEtV5I_RNk Something which, to my knowledge, is being tackled as we speak.

Also, nobody in the foundations community begins a paper without clearly stating that the predictions of QM are amazingly accurate. I think that they would see the QFT calculations that you mention as an incredibly powerful confirmation of that fact. The problem is not in predictive power. The problem is in understanding what QM says about the world.

BEE:...in the end it doesn’t matter. In the end what matters is not what motivates people to study some research question, but only whether it led to something.

Not sure how you cannot not be motivated to study and research more if it has already lead to something else? That's where the organizational part comes in so one can fit different parts to what have been transpiring and come up with a consensus with regard to one's opinion.

You must know that this post would be comment-bait for me, but I'll try to keep it brief.

"Quantum foundations polarizes like no other area in physics. On the one hand there are those actively participating who think it’s the most important thing ever but no two of them can agree on anything."

I think this is a bit unfair. It certainly used to be like that and a lot of older colleagues are still like that, but I like to think that the next generation is a bit more open minded. In past decades, quantum foundations was an extremely fringe research area and researchers were constantly having to justify their existence. There were not that many conferences and people spent most of their time talking to the same small subgroups of people. It is not too surprising that, under these conditions, people would become polarised and cantankerous. These days, quantum foundations is only a very fringe activity rather than an extremely fringe one, so the situation is a bit better. Students are more likely to be well-versed in the full spectrum of approaches and less liable to fall foul of the problem of quantum jumps (which of course refers to the fact that quantum foundations researchers are always jumping to conclusions).

Of course, if you go to a conference organised by a fairly old-school faction of the quantum foundations community then you should not be too surprised if it reflects the old way of doing quantum foundations. I warned you about that on Facebook before you went.

"“Shut up and let me think” is the title of an essay by Pablo Echenique-Robba"

I have to say that this essay has everything I could possibly want in a rant about why we should care about quantum foundations, and also note how open minded he is about pursuing different directions. An example of new-school attitudes in quantum foundations.

"I do have my issues with much of what I’ve seen in quantum foundations. To begin with, most of it seems to be focused on non-relativistic quantum mechanics."

That is very unfair for two reasons. Firstly, it applies mainly to those who have been trying to construct realist accounts of quantum theory along traditional lines, e.g. Bohmians, spontaneous collapse theories etc. Most of the current effort within these communities is focussed on relativistic quantum field theory. Many, but not all, of these approaches introduce a preferred frame, but that seems necessary because of Bell's theorem (unless of course you adopt superdeterminism or the like).

Secondly, and more importantly, in the kind of approach that I favour, I reject the distinction you are trying to make. There is not an activity called interpreting quantum field theory that is distinct in any way from interpreting nonrelativistic quantum theory. There is just the more general issue of interpreting quantum theory, by which I mean the abstract structures of Hilbert spaces, operators, unitaries, etc. which are common to all quantum theories. In fact, the toy models we consider involving qubits and the like are much closer to being a discretization of the quantum field theory state space, in which you have a tensor product of local field operators, than they are to nonrelativistic quantum mechanics. You won't see a nonrelativisitic position operator or a p^2/2m Hamiltonian in any of my talks, and I think it is a mistake to think that the correct interpretation of quantum theory will depend on such details.

"I’m among those who feel comfortable with a mathematical framework that “just” delivers results and that can be used to describe nature. To me science is “just” about making good models."

I agree with you that it is ultimately about exploring all possible routes to come up with the next physical theory. However, I think this is a little bit naive. We should care that our models provide explanations as well as just describing nature. It would take too long to define "explanation" precisely, but it is easy to give an example of what the lack of an explanation looks like. Suppose you had a database in which everything that has happened and ever will happen in our universe was recorded, and some sort of Google-like interface to query it. That would be just as good at describing nature as any mathematical model you could come up with; better even because its results would be more precise. Such an oracle is obviously not a scientific theory and I would argue that the missing component is explanatory power. What many in the quantum foundations community think is that current quantum theory is lacking explanations of some key phenomena. I would point to the example of Bell's theorem, i.e. the purported existence of nonlocal influences that cannot be used to signal, as an explanatory gap that needs closing. I hope that looking at such things will help to formulate future physical theories, but I also think that trying to close the gap is worthwhile even if that does not happen.

Physical theory tolerates any Wesley Crusher abomination that is rigorously derived from orthodoxy. Physics, all sciences, have been remarkably not correct enough since Newton. Physics does not swallow its pride. Contemporary theory cannot be wrong in origin! 20th century seven- and eight-figure mass slaughters (Armenia, Germany, Manchukuo, USSR, PR China, Vietnam, Khmer Rouge, North Korea...) suggest if Hell is anything, it is a belief system.

@msleifer,"What many in the quantum foundations community think is that current quantum theory is lacking explanations of some key phenomena. I would point to the example of Bell's theorem, i.e. the purported existence of nonlocal influences that cannot be used to signal, as an explanatory gap that needs closing. I hope that looking at such things will help to formulate future physical theories, but I also think that trying to close the gap is worthwhile even if that does not happen."

I know I'm belaboring a point, so it pains me to do it, but there are theories out there for this. Unfortunately science fiction appropriated Einstein-Rosen bridges before physics had a chance to take them seriously. Now physicists are afraid to use it in their theories because they are afraid of being tarred with that paint brush.

However there is no reason to be afraid as there is good reason to believe something similar is occuring with the Bell inequalites. No one ever said that an Einstein-Rosen bridge had to always be stable. And if one was created on a small enough scale without the energy to produce stability wouldn't it act exactly like quantum entanglement and produce the Bell inequalities?

I know I'm repeating myself so I apologize. It's just that when I've mentioned this in the past there is never any response one way or another and it doesn't seem to be getting through or even heard. I say this because the physics question my "thought experiment" tried to answer keeps getting asked with no reference to this as a possible answer to the Bell inequalies without signaling.

Of course it could be wrong and I don't deny that, but the deafening silence this idea brings forth makes me wonder if its just too powerful an idea and coming from the wrong source to be taken seriously in the present socialogical state of physics.

Matt Leifer already basically made most of the points that I wanted to make better than I would have made them, but I think that the idea that quantum mechanics as it stands, and as it is understood, lacks a key explanatory element is one that bears repeating.

If I gave you a black box that implements the correct theory of quantum gravity such that you could input physical situations, getting out predictions, would you really be 'pure phenomenologist' enough to be happy with this? Wouldn't you want to know, or at least catch a glimpse of, how the box does it?

For me, that's really why I got into this whole science business: I don't just want to be able to predict, I also want, as much as possible, to understand. Now it might be the case that that's not in the cards---I might be too stupid, or the human mind in general might not be up to it, or there's nothing there to be understood. But I'd at least want to give it my best shot.

There's also an underappreciated aspect of foundational work that maybe is closer to the heart of those who don't share my (admittedly perhaps somewhat romantic) views: it can pave the way for finding new (and better) theories. Consider the Minkowsky formulation of special relativity: in principle, you could do SR without reference to spacetime geometry, introducing a Lorentz ether, the way Bell recommended teaching it---it's ultimately just a different interpretation. However, at least to me, the way from there to general relativity seems much harder.

That's not even considering all the important and intriguing technical advances foundational work has brought with it, starting of course with Bell's theorem, from which basically the whole of quantum information theory flows, on to decoherence (I've read Zeh relate the story of how difficult it was to get his original paper published somewhere), towards more recent things like quantum De Finetti and SIC-POVMs, etc.

Ultimately, however, I think the justification to do quantum foundation is quite divorced from these considerations; it's the same as for climbing a mountain or painting a picture: we don't actually evaluate our actions by the ends they lead to. We only do that when we want to claim that something somebody else is doing isn't worth doing.

I perhaps thought you had read my previous comments on a different thread here on Bee's blog. An ER bridge cab be unstable due to lack of energy input in the creation of the bridge. I.e. it would maintain correlation between particles until the first measurement was made. The bridge would then collapse due to its unstable nature and the energy disturbance required for measurement. Somewhat like a person stepping into a small whirlwind will destroy the whirlwind rather than spin the person. It's no different in principle from the act of measurement.

Again, apologies to others here who may have read my previous comments.

It's not good enough to say that you don't believe in naturalness. It's not about aesthetics.

Naturalness is a consequence of the Effective field theory in the Wilsonian sense. It dictates that you can't have masses in the effective action. Now if masses are allowed by symmetry (like in Higgs) you must rearrange the cutoff accordingly. If you don't find new physics that will remedy the problem in the new cutoff it means that your theory is not natural.

Again this is not an aesthetic or a belief issue, you have to characterize your theory as unnatural i.e. you don't have a choice.

So you must accept that the corresponding fine tuning is unnatural and then you must find the mechanism that will explain the unnatural fine tuning e.g. multiverse.

I think we had an exchange about this previously. What I mean when I say that I don't believe in naturalness is that I don't have any problem to accept finetuning without any further explanation. I also told you previously that I don't think a theory of 'everything' must explain all measured parameters in the standard model. It's perfectly fine by me if they just have some, however 'finetuned', value. Best,

"Many, but not all, of these approaches introduce a preferred frame, but that seems necessary because of Bell's theorem (unless of course you adopt superdeterminism or the like)."

Which is essentially what I was saying, if not as directly :p

"I think this is a bit unfair. It certainly used to be like that and a lot of older colleagues are still like that, but I like to think that the next generation is a bit more open minded."

I certainly hope so...

" I think this is a little bit naive... Suppose you had a database in which everything that has happened and ever will happen in our universe was recorded, and some sort of Google-like interface to query it. That would be just as good at describing nature as any mathematical model you could come up with; better even because its results would be more precise. Such an oracle is obviously not a scientific theory..."

You might call my opinion 'naive', but then it's my opinion. I would say such an oracle just makes science an entirely superfluous enterprise, in fact it makes life itself superfluous. The "Explanations" that you ask for are a means to an end. If you already have that end, what do you need the means for? Think of this from an evolutionary pov, then you'll see what I mean. I don't ask you to agree, but I want to ask you to consider my opinion for a moment before discarding it as blue-eyed. Best,

"If I gave you a black box that implements the correct theory of quantum gravity such that you could input physical situations, getting out predictions, would you really be 'pure phenomenologist' enough to be happy with this?"

No, but that's because I would be afraid that your black box breaks and we wouldn't be any wiser. If you give me a construction instruction for the box, so that I can rebuild it myself, fine by me. I would go ahead and see if there's any phenomenology of quantum gravity that we can possibly use to improve life on planet earth. In fact, I think that's probably how science will be done in the future because sooner or later human cognition will just not be able to comprehend possible 'explanations'. Then we can either try to upgrade the human brain or, if we don't manage to do that, replace mathematical models with models much like your black box. (Think about it. That's pretty much already how it works in some areas of physics. Think turbulences for example.)

Besides, I'm not a 'pure phenomenologist' if there is any. (just look at my publ list. or the archives of this blog.). I understand perfectly well what you're saying. It's just that when you have to put me into a drawer, that's the one I'll be least uncomfortable in. As I wrote in my post, I might not share your desire for deeper 'explanations' of, actually I'm not sure of what, but if that's your motivation then that's your motivation. Best,

Well, what that graph shows is that, embarrassingly, people in your community haven't managed to settle on any interpretation, not to mention explanation. That's not the same as claiming that our understanding of quantum theory is 'embarrassingly poor'. Which, given everything that we have successfully explained by it, is, I maintain, an exaggeration. Best,

A nice piece which I’m not certain represents as a admission of the value of such endeavor or a call for a truce :-) From my perspective with now having been interested in quantum foundations for more than 20 years I would say the greatest benefit it’s provided is to have it made clear as to precisely why Feynman claimed that no one truly understands it; which for me is reason enough to continued to try.

“Here are some words which, however legitimate and necessary in application, have no place in a formulation with any pretension to physical precision: system, apparatus, environment, microscopic, macroscopic, reversible, irreversible, observable, information, measurement.”

The concepts 'system', 'apparatus', 'environment', immediately imply an artificial division of the world, and an intention to neglect, or take only schematic account of, the interaction across the split. The notions of 'microscopic' and 'macroscopic' defy precise definition. So also do the notions of 'reversible' and 'irreversible'. Einstein said that it is theory which decides what is 'observable'. I think he was right - 'observation' is a complicated and theory-laden business. Then that notion should not appear in the formulation of fundamental theory. Information? Whose information? Information about what?”

“On this list of bad words from good books, the worst of all is 'measurement'. It must have a section to itself.”

"What I mean when I say that I don't believe in naturalness is that I don't have any problem to accept finetuning without any further explanation"

How is that different from religion. Only Religions accept unnatural finetuning (Wilson effective theory says it is unnatural not me) without any further explanation, and even religions attribute it to God:-).

Just like every physical theory ever discovered, QM is utterly wrong - not on the details - but on the principles. Think Newton's instant gravity.

If you accept that, then you know that the logical consequences of it are useless. For instance using the math of (or rather lack of a math) for collapse leads one to the many worlds theory - which is certainly wrong if QM is not the last word. In other words, if or rather when someone gets a description of collapse, the MWT dies instantly.

Presumably, it is a reference to the Nazi times. Sabine wasn't even born until decades after then. So, apparently you believe that guilt is somehow inherited. There are at least two problems with that. First, how do you know her ancestors were guilty? Second, this would assume that guilt is somehow passed from generation to generation, like original sin or whatever. This is more bizarre than even the racist ideologies of the Nazis themselves. Or do you think that a race is somehow collectively responsible for certain things? Need I remind you that that is one of the stupid beliefs the Nazis had about the Jews?

If you want to call laws of nature "gods" then that's fine by me. If it's self-consistent, in agreement with observation, doesn't add unnecessary clutter (multiverse, anybody?), and is subject to revision when new data is obtained, then you can accuse me of being "religious" if you want, though I would argue it's actually science. But why fight about words?

I've said it before, but I'll try it again. The appearance of "unnatural", "finetuned" or "unexplained" parameters is to me just another postulated axiom of the theory. Regardless of what theory you fancy (I gather you like string theory), it comes with many axioms already. Which number of axioms do you think is appropriate and why?

Look, I have some sympathy of course for the desire to just have as few axioms as possible. What I am saying is simply that I do not know of any reason why it should not be more than N, whatever N is (seeing that normally nobody starts with a list of axioms), or why parameters are not allowed axioms, or why I should believe that a fundamental theory must be able to explain the values of parameters rather than them being evidence-based input, as is the mere existence of our universe. Best,

If you want to call laws of nature "gods" then that's fine by me. If it's self-consistent, in agreement with observation, doesn't add unnecessary clutter (multiverse, anybody?), and is subject to revision when new data is obtained, then you can accuse me of being "religious" if you want, though I would argue it's actually science. But why fight about words?

The reason not to use this terminology is that these words already have meanings. Yes, it might be self-consistent, but it wouldn't be useful. Instead of "17" you could say "George", 1,423,454,33 could be "Karin" and so on. Mathematics would still work, but what's the point? (Borges wrote a story along these lines.)

Yeah, in principle I agree. It's not like I'm suggesting we call Maxwell's equations Zeus and the gauge/gravity correspondence Thor, or that I think it's likely to happen, it's just that on some level I really couldn't care less what the feck it's called. Best,

I believe that Feynman's advice was directed towards graduate students. Paraphrasing it, what he was saying was "don't work on quantum foundations; it'll take you forever to get your thesis". He worked on quantum foundations himself while I was an undergrad at Caltech—I went to a talk he gave about this. Specifically, he looked at all the hypotheses that went into Bell's theorem and realized that an implicit assumption was that all the probabilities were negative. You can find his paper on negative probability, but I don't believe the Bell's theorem motivation is given in it, presumably because he couldn't find a way to use negative probabilities to get around non-locality.

Getting those rants out of Lubos is another good reason for keeping up work on quantum foundations... Though really, Pablo Echenique-Robba, you definitely should be reprimanded for having such a 'contrived' name.

First, your picture is rotated 90 degrees in Firefox as well. You may want to investigate where is the fault with this. :-)

Second, while I believe that the research about the foundations and interpretations of QM is justified and should be done, I am also very disappointed with the ways it is being pursued. Agreeing with your statement

"If you can’t make it Lorentz-invariant and second quantized I don’t know why I should think about it"

I would suggest an even stronger version: if you can't make it diffeomorphism-invariant and second-quantized, I don't know why I should think about it.

Lorentz-invariance is a mirage made by people who think that gravity can be treated as a spin-two field in flat spacetime. But gravity just isn't like that, and in order to discuss quantum gravity, one really needs a formulation of QM which does not depend on Minkowski (or any other) geometry.

This in turn means formulating QM without the concepts such as coordinates and momenta, energy, time, and maybe even probability and unitarity. What is left after one disposes of these? As far as I can see, the whole QM essentially reduces to the superposition principle and the measurement problem. ;-)

So the real thing would be to formulate QM in a diffeomorphism-invariant way, which is --- alas --- something that very few people (if any?) are trying to do. Everything else regarding interpretations and foundations of QM is practically a waste of time, IMNSHO.

The progress in understanding QM must go hand in hand with the progress in understanding quantum gravity. Otherwise one gets distracted by all those details which actually have no place in a tentative QG model, and therefore in QM as well.

Was there anything on the conference which at least remotely resembles a diff-invariant formulation of QM?

Dear Sabine, apologies, it was intended as a joke. Like your "spot the American" joke about Laurence Krauss. Forget it.ps: to avoid all possible misunderstandings: I thought the Krauss joke was hilarious. Others might not, however.

"I would suggest an even stronger version: if you can't make it diffeomorphism-invariant and ..., I don't know why I should think about it."

Edward Witten seems not to agree:"in my opinion, ... exotic spheres are the only gravitational instantons for which there is a sound basis within the presently understood framework of physics. ... similar remarks can be made about gravitational solitons.

For details, see:http://projecteuclid.org/DPubS?service=UI&version=1.0&verb=Display&handle=euclid.cmp/1103943444

See also:http://loyno.edu/~brans/aei/ex-talk-aei-2007-amended-08.pdf

At least thinking about non-diffeomorphic spacetimes seems to be worth while to me ...

that "mantra" is not feynman's. it's a sentence used by david mermin to describe the attitude of the copenhagen interpretation. see this: http://scitation.aip.org/content/aip/magazine/physicstoday/article/57/5/10.1063/1.1768652

I like the concept: "Shut up and let me think." All work that I have done on foundations has been motivated by a simple dictum: Hypotheses on the physical interpretation are really only useful to the degree that they determine the structure of theories. In my view, this is where many folks go astray. If your alternative view of physical interpretation does not determine a theory which is different in some way then: Why do it?

Hi Kingsley,I'm assuming (perhaps wrongly, let me know) that since you are addressing Bee with this comment and not as an answer to my long reply to you that you are trying to say something under the radar. My guess would be that it is to undermine my comment to you without having to answer my reply.

If you think that my theory is an alternative view of physical interpretation that does not change anything then you are wrong. If quantum entanglement weak decay is the reason for decoherence after measurement then it would change everything. If my interpretation was right it would mean that by strengthening the path of quantum entanglement along the entire path, rather than just the initial separation point of spin sharing, then it might be possible to avoid decoherence after measurement. But I wish you had had the courage to just to address your comment to me, yea or nea, as I did to you.

First at the IOP anniversary issue together with Preskill representing as a top physicist the whole QG field :

“We thought long and hard about the precise wording for the questions, and have invited five top physicists to explain the importance and significance of each. Find out more from Catherine Heymans, Adam Frank, Ray Jayawardhana, Sabine Hossenfelder and John Preskill.”

Then in FT:

‘The next big names in physics’“…she is a rising leader in quantum gravity research, a young field that explores the interplay between the theory of general relativity and quantum mechanics.”

Wow! And again Wow!

What is happening here Sabine? I think as your readership for so many years we deserve some explanations and an update on the status of your physics carrier :-)

I don't know. It seems like a coincidence to me. I interpret it as a side effect of having been around for a while (read: I'm getting old). Also, the FT text is of course nonsense as qg is hardly a 'new' field. It seems to me like they took this from my institute webpage and then dropped the word 'phenomenology'. Best,

I hope it's not too late to get into this discussion. I wanted to read Pablo's paper before responding, and now I feel prepared. (Thank you, Pablo, for a very interesting and enjoyable exposition of this most important issue.)

I want first to express my dismay at the impression I get from Pablo, and many others as well, that Bohr was a positivist and that the Copenhagen interpretation represents an essentially positivist "interpretation" of Quantum physics. (E.g., the notion that meanings don't matter and all we really need to do is experiment and calculate.) In my view nothing could be farther from the truth.

As I see it, Bohr was not only a physicist but a philosopher and not only a philosopher but in fact a metaphysician -- who transformed metaphysics.

And imo if we really want to get down and truly explore the foundations of quantum physics, we have to leave physics completely and move into the dark, mysterious realm of metaphysics. Or, more properly, what could be called "anti-metaphysics." And it is this I miss when I read the various attempts of physicists to "explain" the "meaning" of quantum physics. Because the study of phsyics per se leaves one totally unprepared for dealing with such a topic. (continued in my next comment)

As I see it, the most fruitful way of approaching this topic is not via physics, because imo it isn't really a problem of physics at all, but a more fundamental problem concerning questions such as: limitations of our ability to represent the world around us; the necessity in both science and artistic naturalism of strictly separating the observer from the observed; the special problems that arise when we attempt to carry any process to its limit, etc.

These are fundamental problems, not only of epistemology, but semiotics, phenomenology (pace Bee), and, yes, metaphysics, because there is no limit to how deep they go, or to be more precise: how deeply they cut.

From the Stanford Encyclopedia of Philosophy:

"In general, Bohr considered the demands of complementarity in quantum mechanics to be logically on a par with the requirements of relativity in the theory of relativity. He believed that both theories were a result of novel aspects of the observation problem, namely the fact that observation in physics is context-dependent. This again is due to the existence of a maximum velocity of propagation of all actions in the domain of relativity and a minimum of any action in the domain of quantum mechanics. And it is because of these universal limits that it is impossible in the theory of relativity to make an unambiguous separation between time and space without reference to the observer (the context) and impossible in quantum mechanics to make a sharp distinction between the behavior of the object and its interaction with the means of observation."

One could make a similar statement regarding, say, the relation between foreground and background in the pictorial arts, which seemed unproblematic until certain modern artists demonstrated that, as with the particle/wave duality, any portion of a canvas could be seen as either foreground or background but not both at the same time (as in the rabbit/duck "illusion").

What Bohr the philosopher tells us is that the problem of understanding the "meaning" of quantum physics cannot be separated from more fundamental problems associated with representation in general.

Thus: “There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature...”

What this implies is that a field such as semiotics, which examines the ways in which we represent the world around us, could in fact be considered as in some sense prior to physics. Though in order to do justice to the radical nature of Bohr's thought, we would need a semiotics filtered through the critical lens of post-structuralism or, more accurately, deconstruction. In this regard, I highly recommend a very insightful book by Arkady Plotnitsky: "Complementarity: Anti-Epistemology after Bohr and Derrida."

I recommend also the excellent treatment of the Copenhagen Interpretation to be found in the Stanford Encyclopedia, to be found online here: http://plato.stanford.edu/entries/qm-copenhagen/

A comparison of non-relativistic quantum mechanics to breeding better horses in order to improve traffic would be sensible if horse riding applied to the overwhelming majority of ordinary motorists. Unfortunately, it does not.

By the way, this post made an appearance at the currently ongoing DPG spring conference in Berlin today, in Holger Müller's plenary talk on "Reverse Engineering Quantum Theory" (abstract here: http://www.dpg-verhandlungen.de/year/2014/conference/berlin/part/pv/session/2/contribution/1). In case you'd like to know the circles your words make. :)